As this type of a conventional steering lock apparatus, there is one disclosed in Japanese Patent Laid-Open Publication No. 2004-231122. As shown in FIG. 1 and FIG. 2, this electric steering lock apparatus 100 includes a frame 101 and a cover (not shown) that covers an upper surface of this frame 101. The frame 101 includes a frame body 102 and a rod guide portion 103 protruded from this frame body 102.
On an upper surface of the frame body 102 covered with the cover, there are housed: a drive motor (not shown) controlled by an operation for an ignition key by a person who carries an electronic key; a cam member (not shown) that rotates by drive of this drive motor; and the like. In the rod guide portion 103, a lock rod 105 in which an upper end side protrudes on the frame body 102 is housed so as to be freely movable. This lock rod 105 is composed of an upper rod portion 105a and a lower rod portion 105b coupled thereto, and moves, by the rotation of the above-described cam member (not shown), to a locked position (position of FIG. 1) where a steering shaft 110 is locked and to an unlocked position (position of FIG. 2) where the rotation of the steering shaft 110 is permitted.
On a base end spot of the rod guide portion 103 and a middle spot of the upper rod portion 105a of the lock rod 105, weak portions 106 and 107 thinner in thickness than other spots are provided.
A deadlock mechanism 120 includes: a lock pin 121 housed closer to a tip end side of the rod guide portion 103 than the weak portion 106 thereof; a first spring 122 that urges the lock pin 121 in a protruding direction; a lock control member 123 that regulates movement of the lock pin 121 in a protruding direction; a second spring 124 that urges the lock control member 123 upward; a stopper 125 that is fixed to the frame body 102 and regulates upward movement of the lock control member 123; and first and second engagement grooves 126 and 127 formed on the upper rod portion 105a. 
In the above-described configuration, when the person who carries the electronic key turns the ignition key from an OFF position to an ON position in order to run a vehicle, the lock rod 105 moves by the drive of the drive motor (not shown), and the electric steering lock apparatus 100 turns from a steering locked state of FIG. 1 to a steering unlocked state of FIG. 2. Thereafter, when the ignition key is rotated to an engine start position, an engine is started. When an impact with a predetermined value or more is applied to the rod guide portion 103 and the lock rod 105 as a result of that the vehicle radically slows down during the running thereof and the knees of a driver collide with the electric steering lock apparatus 100 in this case, the rod guide portion 103 and the lock rod 105 are cut off at the spots of the weak portions 106 and 107, whereby an impact received by the driver is reduced.
Then, when the rod guide portion 103 and the lock rod 105 are cut off at the weak portions 106 and 107, as shown in FIG. 3, the stopper 125 comes not to regulate the lock control member 123, the lock control member 123 moves upward by spring force of the second spring 124, and the lock pin 121 is engaged with the second engagement groove 127 of the lock rod 105. In such a way, the unlocked state of the steering shaft 110 is held even after the electric steering lock apparatus 100 is cut off, and driving safety is ensured.
Moreover, when the person who carries the electronic key returns the ignition key from the engine start position to the OFF position in order to park the vehicle, the engine is stopped, and in addition, the lock rod 105 moves by the drive of the drive motor (not shown), and the electric steering lock apparatus 100 turns from the steering unlocked state of FIG. 2 to the steering locked state of FIG. 1.
In this steering locked state, when a person who aims to fraudulently unlock the electric steering lock apparatus 100 attacks the same from the outside, and the impact with the predetermined value or more is applied to the electric steering lock apparatus 100, the rod guide portion 103 and the lock rod 105 are cut off at the spots of the weak portions 106 and 107. Then, as shown in FIG. 4, the stopper 125 comes not to regulate the lock control member 123, the lock control member 123 moves upward by the spring force of the second spring 124, and the lock pin 121 is engaged with the first engagement groove 126 of the lock rod 105. In such a way, the locked state of the steering shaft 110 is held even after the electric steering lock apparatus 100 is cut off, and accordingly, the electric steering lock apparatus 100 is excellent in stealing prevention properties.